It is hoped that organic semiconductors will be used for application to solar cell devices, such as, organic thin-film solar cells, light-emitting devices, and optical sensors. Specifically, for example, if polymers can be used as organic semiconductor materials, it becomes possible to form an active layer by use of a low-cost coating method. Meanwhile, in view of the energy problem and the reduction of CO2 emission, there is rapidly increasing demand for solar cells, which are regarded as one of the clean energy sources with low environmental load. However, although silicon-type solar cells are popularly used now, their efficiencies are about 15% and their production costs are difficult to reduce. Further, although CdTe solar cells are known as low-cost solar cells, they use Cd, which is a toxic element, and hence it is feared that they may cause environmental problems. Under these circumstances, there is increasing hope for developing organic thin-film solar cells, which are harmless next-generation solar cells of low cost and of high energy-conversion efficiency.
However, there are two large problems in realizing organic thin-film solar cells. One is to improve the power generation efficiency. In view of that, researches have been made on various polymeric organic semiconductors usable for organic thin-film solar cells. As a result, it has been found that some polymers enable organic thin-film solar cells to have as high a conversion efficiency as 7% or more, which is a value at the top level in the world. Known examples of them include: poly{4,8-bis(2-ethylhexyloxy)benzo-[1,2-b;4,5-b′]-dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxycarbonyl)-5-fluoro-thieno[3,4-b]thiophene-2,6-diyl} (hereinafter, referred to as “PTB-7”); and a polymer (referred to as “PBDTTT-CF”) derived from PTB-7 by replacing the ester substituent group (i.e., 2-ethylhexyloxycarbonyl) at the 4-position of the thieno[3,4-b]-thiophene ring with a strong electron-withdrawing carbonyl group (i.e., n-heptylcarbonyl) so as to improve the open circuit voltage (Voc). However, those polymers are obtained by very complicated synthetic routes in which monomers are synthesized through many steps.
In order to reduce the steps for synthesis, development has been made to provide poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b′]-dithiophene-2,6-diyl-alt-4-(2-heptylcarbonyl)}thieno-[3,4-b]thiophene-2,6-diyl} (hereinafter, referred to as “PDBTTT-C”), in which fluorine at the 5-position of the thieno[3,4-b]thiophene in PBDTTT-CF is replaced with hydrogen. However, that polymer has a problem of slightly lowering the conversion efficiency. It is hence also known that PDBTTT-C is modified so as to improve the conversion efficiency by replacing 2-ethylhexyloxy groups at the 4- and 8-positions of the benzo-[1,2-b;4,5-b′]-dithiophene ring with 5-(2-ethylhexyl)-thienyl groups.
Those polymers indicate that the conversion efficiency of solar cell greatly depends on side chains of the polymers. Accordingly, for the purpose of improving the performance of solar cells, it is necessary to optimize not only the polymer skeleton but also the side chain and therefore to make many trials and errors.
The other problem in developing organic thin-film solar cells is to expand the lifetime, and it is known that photo-stable active materials (i.e., donor and acceptor) are indispensable for expanding the lifetime of solar cell.